1. Field of the Invention
The present invention relates to an optical receiver circuit using an optical amplifier as a preamplifier.
2. Description of the Related Art
As is already known, an optical fiber amplifier using an optical fiber doped with rare earth elements such as erbium (Er), etc., as an optical amplifying medium has the characteristics of low noise, high gain and a good matching with a transmission fiber. Accordingly, research and development in the field has been actively pursued since it promises a simple and reliable method of increasing the transmission distance. Particularly, an optical receiver, employing an optical fiber amplifier as a preamplifier, greatly improves sensitivity and dynamic range.
FIG. 1 shows the structure of a conventional optical receiver. The optical receiver comprises an optical receiver circuit 600 and a data regeneration circuit 700.
An optical signal input to the optical receiver circuit 600 is supplied to a gain controlled optical amplifier 1, which functions as a preamplifier. In the gain controlled optical amplifier 1, for example, a pump laser drive current is changed, so that an arbitrary gain can be set. Then, an optical input signal is amplified by the set gain so as to be coupled to an optical detector 2 as an optical output signal.
An inverse bias voltage is applied onto the optical detector 2 by a bias circuit 3, and the optical output signal is converted to an electrical signal. The electrical signal is amplified up to a necessary level by an equalization amplifier 4. The output signal of the equalization amplifier 4 is supplied to the data regeneration circuit 700 as a received signal, and a peak detection circuit 7.
The peak detection circuit 7 detects a peak level of the received signal, and a peak detection signal obtained by the circuit 7 is supplied to an error amplification circuit 9. The error amplification circuit 9 obtains an error of the peak detection signal against a predetermined reference level. Then, the error amplification circuit 9 amplifies the error to be output to a gain control circuit 10 as an error signal.
The gain control circuit 10 increases or reduces the pump laser drive current of the optical fiber amplifier 1 such that the error signal becomes zero, thereby the gain of the optical fiber amplifier 1 is controlled. As mentioned above, in the optical receiver circuit 600, the gain of the optical fiber amplifier 1 is increased or reduced in accordance with the change of the peak level of the received signal, so that the peak level of the received signal is stabilized.
The received signal stabilized by the optical receiver circuit 600 is supplied to the data regeneration circuit 700 which consists of a clock extraction circuit 5 and a decision circuit 6.
The clock extraction circuit 5 extracts a clock signal for data decision from the received signal. Then, the extracted clock signal is sent to the decision circuit 6. The decision circuit 6 determines the level of the received signal using the extracted clock signal so as to regenerate a data signal.
The following will explain an operation of a gain control of the above-structured optical receiver circuit 600 with reference to FIGS. 2 and 3.
In FIG. 2, the solid line shows an optical fiber amplifier gain (dB) 1 against the input optical power level (dBm) of the optical receiver circuit 600, and the dot-dash line shows the optical receiver circuit overload limit is equal to the maximum receiving optical level.
As is obvious from the figure, the optical fiber amplifier 1 is controlled such that the gain becomes small if the input optical power level increases, and the gain becomes large if the input optical power level decreases. Due to this, even if the level of the optical input signal changes, a stabilized received signal can be obtained by the optical receiver circuit 600. As a result, data can be surely decided and regenerated by the data regeneration circuit 700.
However, in the above-structured optical receiver circuit 600, in a case of an input signal loss in which the level of the optical input signal becomes lower than a certain level, for example, disconnection of an optical signal input connector, the gain control works such that the gain of the optical fiber amplifier 1 becomes maximum.
Under the maximum gain state, a gain of the optical fiber amplifier 1 does not become constant against the input optical power level by a saturation characteristic, which the amplifier has, as shown in a broken line of FIG. 2. In this state, the output of the optical fiber amplifier 1 exceeds the optical receiver circuit overload limit 600 as shown in the dot-dash line of FIG. 2.
Due to this, as shown in FIG. 3, if the input optical power level abruptly increases higher than a reference level P4 from the input signal loss level, the output of the optical fiber amplifier 1 exceeds the maximum receiving optical level of the optical receiver circuit 600 since the gain of the optical fiber amplifier 1 becomes maximum. At this time, for example, an optical signal having excessive optical power is made incident onto the receiving optical element 2, there is the possibility that the receiving optical element 2 will be damaged.
Also, if the optical power level of the optical input signal is reduced, the rate of optical noise in the optical output signal is increased in accordance with increase in the gain of the optical fiber amplifier 1. Due to this, if the gain of the optical fiber amplifier 1 is controlled such that the peak level of the received signal is fixed by the the gain control circuit 10, the optical power level of the optical output signal increases as the optical power level of the optical input signal decreases. Therefore, if the optical power level of the optical input signal continues to decrease, the output optical power level of the optical fiber amplifier 1 continues to increase and exceeds the maximum receiving optical level of the optical receiver circuit 600. As a result, an abnormal operation may be generated in the circuit or the circuit may be damaged.
The prior art of FIG. 1 explained the case of the output level detecting system of the optical fiber amplifier 1 after the conversion from optical signal to the received signal, which is converted to the electrical signal. However, there is a system in which a part of the optical output signal of the optical fiber amplifier 1 is divided before the received signal is converted to the electrical signal so as to detect the output optical power level. Even in this system, in a case where the optical signal is input again from the input signal loss, there is generated a problem in which the optical receiver circuit will be damaged or the circuit cannot be started again, depending on the case.